1. Technical Field
The invention relates to radiation sensors of the kind in which a semiconductor device, such as a semiconductor diode or a field effect transistor, is used to detect radiation. The invention is especially, but not exclusively applicable to radiation sensors for dosimeter probes used to monitor levels of radiation during medical procedures, such as the treatment of tumours, and to dosimeters incorporating such sensors. The invention also comprehends semiconductor radiation sensors per se whether for use in such dosimeter probes or elsewhere.
2. Background Art
Dosimeter probes having semiconductor radiation sensors are used routinely in radiotherapy clinics to measure the radiation dose given to patients. The most commonly used electronic patient dosimeters use probes having silicon diodes or silicon metal oxide semiconductor field effect transistors (MOSFETs), otherwise known as an insulated gate field effect transistors (IGFETs), as radiation sensors. For example, U.S. Pat. No. 5,444,254 (Thomson) issued Aug. 22, 1995 discloses a dosimeter probe comprising a radiation sensor in the form of a pair of MOSFETs formed epitaxially on a silicon substrate which is bonded to one end of a flexible electrical connector. A meter attached to the other end of the flexible connector measures the level of radiation and total accumulated radiation. The flexible connector allows the sensor to be inserted, via a catheter, to a position adjacent a site being irradiated.
Whether the radiation sensor is a diode or a FET, it is generally planar and the active detection region is a thin layer, typically less than 1 xcexcm thick, on the surface of a relatively thick silicon substrate, typically about 500 xcexcm thick. In the case of a diode sensor, the thin detection layer is the depletion region of a zero-biased PN junction, which usually has an area of several mm2. Ionizing radiation passing through the silicon junction area during exposure creates charge carriers which are detected as a current in external circuitry. This current is proportional to the dose rate of the incident radiation and the external circuitry measures the current and integrates it over the exposure time to determine the dose.
In the case of a MOSFET radiation sensor, the thin detection layer is the silicon dioxide forming the gate of the MOSFET. Ionizing radiation passing through the gate oxide generates charge, which is trapped. The trapped charge changes the threshold voltage of the MOSFET and the external circuitry determines the change in the threshold as a measure of the radiation dose absorbed by the MOSFET. Typically, the area of a MOSFET radiation sensor is 0.2 mm square on a die that is 1 mm square.
In an article entitled xe2x80x9cClinical dosimetry using MOSFETsxe2x80x9d, Int. J. Radiation Oncology Biol. Phys., Vol. 37, No. 4, 1997, pp. 959-964, Ramaseshan Ramani et al. reported that the sensitivity of a MOSFET sensor of the aforementioned kind can vary by as much as 28 percent according to the angle of incidence of the radiation. While such a radiation sensor may perform satisfactorily in most situations, such anisotropic sensitivity limits its usefulness with new treatment techniques such as intensity modulated radiation therapy (IMRT), conformal radiation therapy and brachytherapy, all of which direct the radiation onto the tumour and the radiation sensor, during treatment, from angles which can vary over a wide range. IMRT, for example, uses radiation beams which are directed at the tumour (and adjacent sensor) by rotating the radiation source through plus and minus 135 degrees. In the case of brachytherapy, seeds of radioactive material are implanted into the tumour in a non-uniform array and the radiation sensor is positioned within the array. It is not practical to determine the orientation of the radiation sensor once it has been inserted, so anisotropic sensitivity of the radiation sensor can lead to inaccurate readings. Even with other forms of treatment, which use a single, fixed radiation angle, it might be desirable to monitor the radiation received by adjacent organs, but this cannot be done accurately in view of the anisotropic sensitivity and the lack of information about the orientation of the radiation sensor.
Anisotropic sensitivity may be exacerbated by the packaging of the sensor, specifically the use of so-called xe2x80x9cbuild upxe2x80x9d material, usually aluminum or brass, around the die. Permanently adding build-up during manufacture would mean that a different dosimeter probe would be required for each photon energy range (e.g. 6-MV, 10-MV, 18-MV) and electron energy level. It is preferable, therefore, to package MOSFET radiation sensors with a minimum of build-up material around the die so that the physicist can add build-up according to the energy at which the dosimeter probe is to be used, allowing the same dosimeter probe to be used for all photon and electron energy ranges. The build-up also contributes to the anisotropic response to radiation. Radiation sensors which use diodes rather than MOSFETs are even more likely to exhibit anisotropic sensitivity because such diodes usually are packaged with more xe2x80x9cbuild-upxe2x80x9d material around them.
The present invention seeks to overcome or at least mitigate these disadvantages of known radiation sensors.
According to a first aspect of the present invention, a radiation sensor comprises an active semiconductor device fabricated upon a surface of a substrate and a body of semiconductor material positioned adjacent said surface but insulated therefrom by a thin film of insulating material, so as to overlie an active region of the semiconductor device, the body of semiconductor material being dimensioned to circumscribe the active region and an area of the substrate surface surrounding the active region, such that, whatever the direction from which the radiation is incident upon the active region, its path through the device before arriving at the active region will have similar characteristics to the path the radiation takes upon leaving the active region.
According to a second aspect of the present invention, a dosimeter probe comprises an elongate flexible conductor strip having a radiation sensor at one end, a connector at an opposite end, and a plurality of conductors connecting the radiation sensor to the connector, wherein the radiation sensor comprises an active semiconductor device fabricated upon a surface of a substrate attached to the flexible conductor strip and a body of semiconductor material positioned adjacent said surface but insulated therefrom by a thin film of insulating material, so as to overlie an active region of the semiconductor device, the body of semiconductor material being dimensioned to circumscribe the active region and an area of the substrate surface surrounding the active region, such that, whatever the direction from which the radiation is incident upon the active region, its path through the device before arriving at the active region will have similar characteristics to the path the radiation takes upon leaving the active region.
Preferably, in embodiments of either aspect of the invention, the arrangement is such that, whatever the direction from which the radiation is incident upon the active region, its path through the device before arriving at the active region will have similar characteristics to the path the radiation takes upon leaving the active region.
Such a requirement can be met if the composition of the material all around the semiconductor device, and especially on opposite sides of the planar surface and the depletion layer or radiation detection layer, is generally symmetrical for a significant distance each side of the active region.
Hence, it is preferable for the spacing between the body and the juxtaposed substrate surface to be a minimum, for example a few thousandths of an inch, and for the thickness of the body to be substantially the same as the thickness of the substrate.
The radiation characteristics of the material from which the body is made preferably are similar to those of the material from which the substrate is made, so as to make backscatter substantially the same in every direction. Such characteristics may include atomic number. Allowance may also be made for conductors, typically of copper, in the radiation path.
The body need not be connected electrically to the substrate or to the semiconductor device.
The semiconductor device may be a transistor or a diode but, in preferred embodiments of the invention, comprises a MOSFET.